Light measuring device capable of measuring optical power level easily with high accuracy

ABSTRACT

To measure the optical power level of light from an object to be measured, a bias section applies to a photo-detecting section such a reverse bias voltage as makes the current multiplication factor M almost zero to prevent the output current from flowing, and then applies such a reverse bias voltage as makes the M one or more to allow the output current to flow. A processing section determines an offset level from the output of a direct-current amplifying section during an offset data acquisition period that the M is almost zero. The time T0 from when the bias section applies to the photo-detecting section such a reverse bias voltage as makes the M one or more to when the reverse bias voltage is applied to make the M almost zero is determined to be a measurement state. The processing section measures the optical power with a clock faster than time T0. The processing section subtracts the offset level from the measured optical power level to compensate for the offset, thereby determining the correct optical power level of the measured object. This makes it possible to automatically perform offset compensation without shading light before measurement and realize highly accurate measurement.

BACKGROUND OF THE INVENTION

This invention relates to a light measuring device, and moreparticularly to a light measuring device capable of measuring the powerlevel of incident light.

In a light measuring device, such as an optical power meter, formeasuring the power level of the incident light from an object to bemeasured, such as a light source or an optical transmission deviceexternally connected to a photo-detecting system, the offset level ofthe photo-detecting system (e.g., an amplifier) must be compensated for.

Here, the offset level is defined as the output level of thephoto-detecting system on which no light from the object to be measuredis projected.

The accurate optical power of light from the measured object can becalculated by subtracting the offset level from the output level of thephoto-detecting system on which the light from the measured object hasbeen projected.

The subtraction process is called offset compensation.

In the case of conventional light measuring devices, before opticalpower is measured, offset compensation is made according to thefollowing procedure.

First, a shade cover is put on the connector section of the lightmeasuring device to which an object to be measured is connected.

Then, the output level of the light measuring device whose connectorsection has not been struck by light is measured and the measured levelis determined to be the offset level.

Thereafter, the shade cover is removed and an object to be measured isconnected to the connector section of the light measuring device. Then,the light from the object is allowed to strike the connector section.

Then, the previously determined offset level is subtracted from thelevel of the light sensed by the photo-detecting system of the lightmeasuring device at that time. In this way, the resulting level ismeasured as the optical power level of light from the measured object.

The offset compensation is made periodically to reduce measurementerrors.

With the conventional light measuring devices, however, light must beshaded to make the above-described offset compensation. This requiresthe laborious task of putting a shade cover on the connector sectionbefore measurement, measuring the offset level, removing the shadecover, connecting an object to be measured to the connector section ofthe light measuring device, and making the desired measurement.

Furthermore, the conventional light measuring devices prevent thedesired measuring operation from being started immediately.

The causes of preventing the offset level from decreasing to zero withthe conventional light measuring devices are roughly divided into thefollowing two factors:

1. dark current in the photo-detecting element

2. a direct-current amplifier connected to the photo-detecting element

The inventors of the specification have considered the effect of factor1 on the measurement result.

FIG. 2 shows a characteristic of the output current, dark current, andcurrent multiplication factor of an InGaAs APD (Avalanche Photodiode)versus reverse voltage.

In FIG. 2, when the optical power of the measured object is measured(with a reverse bias at a multiplication factor of about one), a darkcurrent of about 10⁻¹⁰ A is generated.

The dark current is added to the received-light current caused by thelight from the measured object. The APD outputs the resulting current.

FIG. 3 shows the calculated measurement error by the dark current inthis condition corresponding to the optical power.

It is seen from FIG. 3 that if the optical power is in the range down toabout -50 dBm, measurements can be made with an error of less than 0.05dB.

Therefore, as shown in FIG. 3, when optical power is measured in therange down to -50 dBm, the necessity for compensating for the offsetlevel due to factor 1 is small. As a result, only the offset level dueto factor 2 has only to be compensated for.

One known light measuring device is an OTDR (an Optical Time DomainReflectometer), which throws light pulse on a fiber to be measured,processes the reflected light (back scattering light or Fresnelreflected light) from the measured fiber as a result of the supply ofthe light pulse, and measures losses or defective points in the measuredfiber.

FIG. 6 shows a general configuration of an OTDR of this type.

The OTDR comprises a timing generator section 21, a light pulse emittingsection 22, a branch section 23, a photo-detecting section 24, adirect-current amplifying section 25, an A/D conversion section 26, aprocessing section 27, and a display section 28.

In the OTDR, on the basis of the control signal from the processingsection 27, the timing generator section 21 outputs a signal to thelight pulse emitting section 22 in a period corresponding to the lengthof a fiber 29 to be measured, that is, in the period T longer than timet required for the reflected light to come back the total length of thefiber 29 since the supply of the light pulse to the fiber 29.

Receiving the output, the light pulse generator section 22 generates alight pulse in each period T.

The light pulse generated in each period T by the light pulse generatorsection 22 is allowed to input the fiber 29 via the branch section 23.

The reflected light returning from the fiber 29 as a result of thesupply of the light pulse is allowed to input the photo-detectingsection 24 via the branch section 23, which converts the light intoelectricity.

The photoelectrically converted signal is converted by the A/D convertersection 26 into a digital signal. The digital signal is inputted to theprocessing section 27.

The processing section 27 performs the process of logarithmicallyconverting the inputted digital signal by sampling the data.

On the basis of the signal processing, a waveform is displayed on thedisplay section 28.

With the OTDR of this type, a light pulse is thrown to the fiber 29 ineach period T (e.g., at intervals of one msec).

Of an N number of (e.g., 5000) data items sampled during one period T,the average value of an M number of (e.g., 20) data items not containingthe reflected light from the fiber 29 is determined to be the offsetlevel.

Then, offset compensation is made by subtracting the offset level fromeach of the N number of data items sampled.

Recent OTDRs are required to have the function of measuring the powerlevel of the incident light from the object, such as a light source or alight transmission device.

In the case of a configuration realizing such a function, thereceived-light power level of incident light is measured without using alight pulse from the light pulse emitting section 22.

For this reason, with the configuration realizing the above-describedfunction, offset compensation for the photo-detecting system must bemade in the OTDR as in generally used light measuring devices.

BRIEF SUMMARY OF THE INVENTION

From the above consideration and analysis, it has become clear that ifoptical power is in the range down to about -50 dBm, the received-lightpower level of the incident light from an object to be measured can bemeasured with sufficient accuracy by compensating for only the offsetlevel due to the factor 2 which is aforementioned.

It is, accordingly, an object of the present invention to solve theproblem of the above-described prior art by providing a light measuringdevice capable of easily measuring, with high accuracy, thereceived-light power level of light from an object to be measured byeffecting offset compensation without shading light before measurementas in the prior art.

According to the present invention, there is provided a light measuringdevice comprising: a photo-detecting section whose output current variesaccording to an applied reverse bias voltage and which receives lightfrom a measured object and outputs a current proportional to theintensity of the light; a bias section which generates a first reversevoltage to prevent the output current of the photo-detecting sectionfrom flowing and a second reverse bias voltage to allow the outputcurrent of the photo-detecting section to flow and applies the firstreverse bias voltage and the second reverse bias voltage to thephoto-detecting section; a direct-current amplifying section whichoutputs a signal whose level is proportional to the output current fromthe photo-detecting section; and a processing section which determinesthe intensity of light from the measured object from a first output fromthe direct-current amplifying section when the bias section iscontrolled so that the first reverse bias voltage may be applied to thephoto-detecting section and a second output from the direct-currentamplifying section when the bias section is controlled so that thesecond reverse bias voltage may be applied to the photo-detectingsection.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinbefore.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1A is a block diagram of a light measuring device according to afirst embodiment of the present invention;

FIG. 1B is a detailed diagram of the bias section in the light measuringdevice in the first embodiment;

FIG. 2 shows a current versus reverse voltage characteristic of anInGaAs APD used as the photo-detecting section of the light measuringdevice of the first embodiment;

FIG. 3 shows an example of the calculated measurement error by the darkcurrent of the InGaAs APD corresponding to the optical power of anobject to be measured;

FIGS. 4A, 4B, and 4C are timing charts to help explain the offsetcompensation operation in the light measuring device of the firstembodiment;

FIG. 5 is a block diagram of a light measuring device according to asecond embodiment of the present invention; and

FIG. 6 is a block diagram of an OTDR taken as an example of aconventional light measuring device.

DETAILED DESCRIPTION OF THE INVENTION

Reference will not be made in detail to the presently preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which like reference characters designate like orcorresponding parts throughout the several drawings.

An outline of a light measuring device according to the presentinvention will be explained by reference to FIG. 1A.

In the knowledge that if optical power is in the range down to about -50dBm, the power level of the incident light from an object to be measuredcan be measured with sufficient accuracy by compensating for only theoffset level due to the factor 2 which is aforementioned, lightmeasuring devices according to the present invention have the followingconfigurations.

A light measuring device according to the first aspect of the presentinvention is characterized by comprising: a photo-detecting section 5whose output current varies with an applied reverse bias voltage andwhich receives light from an object to be measured; a bias section 6which applies a reverse bias voltage to the photo-detecting section insuch a manner that the bias voltage can vary; a direct-currentamplifying section which outputs a signal whose level is proportional tothe output current from the photo-detecting section; and means 8 and 9which determine a first output level outputted from the direct-currentamplifying section when the bias section applies the reverse biasvoltage to the photo-detecting section to prevent the output currentfrom flowing, determine a second output level outputted from thedirect-current amplifying section when the bias section applies thereverse bias voltage to the photo-detecting section to allow the outputcurrent to flow, and determine the optical power level of the object bysubtracting the first output level from the second output level tocompensate for an offset.

A light measuring device according to the second aspect of the presentinvention is characterized in that in the light measuring deviceaccording to the first aspect of the present invention, the bias section6 applies to the photo-detecting section such a reverse bias voltage asmakes the current multiplication factor of the photo-detecting sectionalmost zero to prevent the output current of the photo-detecting section5 from flowing and applies to the photo-detecting section such a reversebias voltage as makes the current multiplication factor of thephoto-detecting section one or more to allow the output current of thephoto-detecting section 5 to flow to enable the measurement of theoptical power level of the measured object.

In the light measuring device of the present invention, means 8 and 9determine the first output level outputted from the direct-currentamplifying section 7 when the bias section 6 applies a reverse biasvoltage to the photo-detecting section 5 to prevent the output currentof the photo-detecting section 5 from flowing.

Furthermore, the means 8 and 9 determine the second output leveloutputted from the direct-current amplifying section 7 when the biassection 6 applies a reverse bias voltage to the photo-detecting section5 to allow the output current of the photo-detecting section 5 to flow.

Then, the means 8 and 9 determine the optical power level of the objectby subtracting the first output level from the second output level tocompensate for the offset.

Accordingly, with the present invention, it is possible to provide alight measuring device capable of easily measuring the power level ofthe incident light from the measured object with high accuracy byperforming offset compensation without shading light before measurementas in the prior art.

Embodiments according to the present invention as described above willbe explained by reference to the accompanying drawings.

FIG. 1A is a block diagram of a light measuring device according to afirst embodiment of the present invention.

As shown in FIG. 1A, the light measuring device comprises an inputsection 1, a timing generator section 2, a light pulse emitting section3, a branch section 4, a photo-detecting section 5, a bias section 6, adirect-current amplifying section 7, an A/D conversion section 8, aprocessing section 9, and a display section 10.

The input section 1 has a fiber measuring mode and an optical powerlevel measuring mode and chooses one of these modes as a result of keyoperation.

The input section 1 outputs the signal corresponding to the chosen modeto the processing section 9 as described later.

The input section 1 also outputs the various measuring parametersnecessary for signal processing to the processing section 9.

On the basis of a control signal Si from the processing section 9, thetiming generator section 2 outputs a timing signal S2 to the light pulseemitting section 3 and A/D conversion section 8.

Being triggered by the timing signal S2 from the timing generatorsection 2, the light pulse emitting section 3 outputs a light pulse in along wavelength band of, for example, 1.3 μm or 1.55 μm.

The light pulse from the light pulse emitting section 3 is emitted viathe branch section 4 to the measured fiber 11 in the periodcorresponding to the length of the fiber 11, that is, in the period Tlonger than time t required for the reflected light to come back thetotal length of the fiber 11 since the supply of the light pulse to thefiber 11.

The branch section 4 is composed of, for example, a directional opticalcoupler and branches to the side of the photo-detecting section 5 thereflected light (back scattering light or Fresnel reflected light) fromthe fiber 11 as a result from the supply of light pulse in the fibermeasuring mode or the incident light from the object to be measured,such as a light source or an optical transmission, in the power levelmeasuring mode.

The photo-detecting section 5 is composed of, for example, InGaAs APD,and has the function of amplifying electrons or holes created by theincident light at a specific current multiplication factor.

The photo-detecting section 5 senses not only the reflected light fromthe fiber 11 separated by the branch section 4 in the fiber measuringmode but also the incident light from the measured object in the opticalpower level measuring mode.

The current multiplication factor M of the photo-detecting section 5 ischanged according to the level of the reverse bias voltage applied bythe bias section 6.

An InGaAs APD having a characteristic of the output current, darkcurrent, and current multiplication factor versus the reverse voltageshown in FIG. 2 is used as the photo-detecting section 5.

When the InGaAs APD is used, it is evident from FIG. 2 that as thereverse bias voltage is made closer to 0 V, the output current scarcelyflows and the dark current also decreases.

Namely, when the InGaAs APD is used, making the reverse bias voltagecloser to 0 V makes the current multiplication factor M of thephoto-detecting section 5 almost 0.

In this case, the offset due to the dark current in the photo-detectingsection 5 has a negligible value as analyzed in BACKGROUND OF THEINVENTION.

In FIG. 2, the fact that making the current multiplication factor Mcloser to 0 decreases the dark current of the APD means that it can notbe compensated the offset level due to factor 1 which is aforementioned.

The photo-detecting section 5 is not restricted to the InGaAs APD andmay be any suitable device, provided that the variation of the reversebias voltage allows almost no output current of the device to flow.

On the basis of a control signal S3 from the processing section 9, thebias section 6 applies a specific reverse bias voltage to thephoto-detecting section 5.

More specifically, when an InGaAs APD is used as the photo-detectingsection 5, the bias section 6, receiving a control signal S3a for anoffset compensation instruction from the processing section 9, appliesto the photo-detecting section 5 such a reverse bias voltage (e.g., inthe rage of 0 V or higher and 13 V or lower) as makes the currentmultiplication factor M almost 0 at which almost no current flows in thephoto-detecting section 5.

In contrast, when receiving a control signal S3b for a measurementinstruction from the processing section 9, the bias section 6 applies tothe photo-detecting section 5 such a reverse bias voltage (e.g., in therange of 16 V or higher and 100 V or lower) as makes the currentmultiplication factor M meet M≧1 to measure the incident light from themeasured object.

The direct-current amplifying section 7 amplifies the output currentfrom the photo-detecting section 5 at a specific multiplication factorand outputs the result to the A/D conversion section 8.

More specifically, the direct-current amplifying section 7 is composedof, for example, a current-voltage converter and a voltage amplifier. Inthe direct-current amplifying section 7, the current-voltage converterconverts the current signal outputted from the cathode of thephoto-detecting section 5 into a voltage signal. The voltage amplifierthen amplifies the converted voltage signal at a specific multiplicationfactor.

At that time, the multiplication factor of the voltage amplifier in thedirect-current amplifying section 7 is varied by the control signal fromthe processing section 9.

In the fiber measuring mode, being triggered by the timing signal S2from the timing generator section 2, the A/D conversion section 8samples the signal amplified at the direct-current amplifying section 7in a specific sampling period and digitizes the sampled signal. Then,the A/D conversion section outputs the sampled digital signal to theprocessing section 9.

Furthermore, in the optical power level measuring mode, the A/Dconversion section 8 converts the signal amplified at the direct-currentamplifying section 7 into a digital signal and outputs it to theprocessing section 9.

The processing section 9 performs the process of logarithmicallyconverting the sampling signal from the A/D conversion section 8 in thefiber measuring mode.

Each data item obtained from the signal processing is displayed as, forexample, waveform data, on the display section 10, such as a CRTdisplay.

Furthermore, the processing section 9 performs the process ofcalculating an optical power level from the digital signal from the A/Dconversion section 8 in the optical power level measuring mode.

The data on the optical power level obtained by the calculation processis displayed on the display section 10.

Moreover, on the basis of the input signal from the input section 1, theprocessing section 9 outputs control signals S1 and S3 (S3a and S3b) tothe timing generator section 2 and the bias section 6.

More specifically, the processing section 9 is composed of, for example,a CPU, and has a timer circuit in it. When the optical power levelmeasuring mode is set, the processing section 9 outputs the controlsignal S3a indicating an offset instruction to the bias section 6 eachtime the time set in the timer circuit has elapsed.

When not outputting the control signal S3a indicating an offsetcompensation instruction, the processing section 9 outputs the controlsignal indicating a measurement instruction to the bias section 6.

At that time, when the fiber measuring mode is chosen from the inputsection 1, the processing section 9 outputs the control signal S1instructing the timing generator section 2 to emit a light pulse.

A series of operations including offset compensation for the lightmeasuring device constructed described above will be described byreference to timing charts as shown in FIGS. 4A, 4B, and 4C.

After the power switch (not shown) has been turned on to start the lightmeasuring device, when the optical power level measuring mode is chosenfrom the input section 1, the light pulse emitting section 3 isprevented from emitting a light pulse and the incident light from theobject to be measured is thrown to the photo-detecting section 5. Then,the photo-detecting section 5 performs photoelectric conversion.

At that time, the processing section 9 outputs control signal S3aindicating an offset compensation instruction to the bias section 6.

When receiving control signal S3a from the processing section 9, thebias section 6 applies to the photo-detecting section 5 such a biasvoltage (0) as makes the current multiplication factor M of thephoto-detecting section 5 almost 0.

This allows almost no output current to flow in the photo-detectingsection 5.

At that time, the output, including dark current, of the photo-detectingsection 5 is amplified at the direct-current amplifying section 7 andthen, together with the offset voltage from the direct-currentamplifying section 7, is converted into a digital signal at the A/Dconversion section 8. The digital signal is then inputted to theprocessing section 9 as offset data in the optical power level measuringmode.

The offset data is stored in, for example, a memory (not shown) withinthe processing section 9.

As shown in FIGS. 4A, 4B, and 4C, the time from when such a bias voltage(0) as makes the current multiplication factor M of the photo-detectingsection 5 almost 0 is applied until such a bias voltage (-V) as makesthe current multiplication factor M of the photo-detecting section 5meet M≧1 is applied is determined to be an offset data acquisitionperiod.

When receiving control signal S3b indicting a measurement instructionfrom the processing section 9, the bias section 6 applies to thephoto-detecting section 5 such a bias voltage (-V) as makes the currentmultiplication factor M of the photo-detecting section 5 meet M≧1 tobring the photo-detecting section 5 into the measurement state.

The output current from the photo-detecting section 5 is converted intoa voltage and the voltage result is amplified at the direct-currentamplifying section 7.

Then, the signal amplified at the direct-current amplifying section 7,together with the offset voltage from the direct-current amplifyingsection 7, is converted into a digital signal at the A/D conversionsection 8. The digital signal is then inputted to the processing section9.

The processing section 9 performs offset compensation by subtracting theoffset data measured and stored at the time of the preceding offsetcompensation instruction (that is, when the current multiplicationfactor M of the photo-detecting section 5 meets M≠0) from the data ofthe digital signal inputted at that time.

The level of the data after the offset compensation is the correctoptical power level of the measured object.

As shown in FIGS. 4A, 4B, and 4C, the time from when such a bias voltage(-V) as makes the current multiplication factor M of the photo-detectingsection 5 meet M≧1 is applied until such a bias voltage (0) as makes thecurrent multiplication factor M of the photo-detecting section 5 almost0 is applied is determined to be allocated to the measurement state.

While the optical power level measuring mode is being chosen, the offsetdata when the current multiplication factor M of the photo-detectingsection 5 becomes almost 0 is acquired repeatedly at intervals of timeT0.

Within time T0, the optical power of the incident light when the currentmultiplication factor M of the photo-detecting section 5 meets M≧1 ismeasured more than once with a faster clock than T0.

Then, the already acquired offset data is subtracted from eachmeasurement data item and the level after the subtraction is displayed.

This reduces the number of times the offset data is acquired to onlyonce, while the processing section 9 measures and displays the opticalpower of the incident light more than once (for example, 50 times)during time T0.

The relationship between the measurement and display of the opticalpower of the incident light and the acquisition of the offset datadepends on the temperature in an environment where the device is used.

For example, when the temperature varies fast, time T0 should be setshorter to acquire the offset data at shorter intervals.

Accordingly, with the first embodiment, simply turning on the powersupply enables the necessary offset compensation in the optical powerlevel measuring mode to be made automatically at regular intervals oftime.

At that time, as the reverse bias voltage applied to the photo-detectingsection 5 is closer to 0, errors in measurement decrease more.

Offset compensation in the first embodiment can be made even when nolight is incident on the photo-detecting section 5.

When the fiber measuring mode is selected from the input section 1, theprocessing section 9 outputs control signal S1 indicating the emissionof a light pulse to the timing generator section 2.

Being triggered by timing signal S2 from the timing generator section 2,the light pulse emitting section 3 emits a light pulse via the branchsection 4 to the fiber 11 to be measured.

The light pulse emitting section 3 emits the light pulse repeatedly inthe period corresponding to the length of the fiber 11, that is, in theperiod T longer than time t required for the reflected light to comeback the total length of the fiber 11 since the supply of the lightpulse to the fiber 11.

The reflected light returning from the fiber 11 as a result of thesupply of the light pulse is allowed to incident on the photo-detectingsection 5 via the branch section 4. Then, the photo-detecting section 5converts the light into electricity.

The photoelectrically converted signal is amplified by thedirect-current amplifying section 7 and is converted by the A/Dconverter section 8 into a digital signal. The digital signal is theninputted to the processing section 9.

The processing section 9 performs the process of logarithmicallyconverting the inputted digital signal by sampling the data.

On the basis of the result of the signal processing, the waveform isdisplayed on the display section 10.

In the above operation, offset compensation in the fiber measuring modeis made as in the prior art.

Specifically, a light pulse is thrown to the fiber 11 in each period T(e.g., at intervals of one msec).

Of an N number of (e.g., 5000) data items sampled during one period T,the average value of an M number of (e.g., 20) data items not containingthe reflected light from the fiber 11 is determined to be the offsetlevel. Then, offset compensation is made by subtracting the offset levelfrom the N number of data items sampled.

FIG. 1B is a detailed diagram of the bias section 6 in the lightmeasuring device according to the first embodiment.

In FIG. 1B, a step-up section 61 raises an input direct-current voltage(+5 VDC) from the input terminal (IN) to an alternating-current voltage(140 VAC) and supplies the alternating-current voltage to a rectifiersection 62.

The rectifier section 62 full-wave rectifies the alternating-currentvoltage (140 VAC) and supplies the rectified voltage to the collector ofa transistor 63.

The emitter of the transistor 63 is connected to a voltage outputterminal (OUT) serving as the bias section 6.

An output voltage sensing section 64 divides the output voltage at thevoltage output terminal (OUT) and outputs the divided voltage to anerror amplifier 65.

The error amplifier 65 compares the output from the output voltagesensing section 64 with a reference voltage (2.5 VDC) from a referencevoltage generator section 66 and controls the base of the transistor 63so that the difference between the output and the reference voltage maybe kept constant.

At that time, the reference voltage from the reference voltage generatorsection 66 is set variably by a digital control variable resistor 67.

The digital control variable resistor 67 is controlled by, for example,a 4-bit control signal from the processing section 9.

As described above, a specific reverse bias voltage can be applied tothe photo-detecting section 5 by causing the digital control variableresistor 67 to set the reference voltage from the reference voltagegenerator section 66 variably.

FIG. 5 is a block diagram of a light measuring device according to asecond embodiment of the present invention.

The light measuring device of the second embodiment is constructed byeliminating the timing generator section 2, light pulse emitting section3, and branch section 4 from the light measuring device of the firstembodiment.

In other words, the light measuring device of the second embodiment isthe same as that of the first embodiment except that the fiber measuringmode is absent.

With this configuration, the optical power level measuring mode in thefirst embodiment can be set when the power supply is turned on.

Therefore, just turning on the power supply enables the optical powerlevel subjected to automatic offset compensation to be measured.

In the first embodiment, the signal at the time when the optical powerlevel measuring mode is chosen has been used as a trigger signal. In thesecond embodiment, the signal at the time when the power switch isturned on has been used as a trigger signal. With these trigger signals,the offset data has been acquired at intervals of time T0 while theoptical power level measuring mode or the power switch is on.

The input section 1 may be provided with a separate key to select theoffset mode. Then, the processing section 9 may be allowed to output anoffset compensation instruction or a measurement instruction to the biassection 6, depending on whether the signal at the time when the offsetmode is chosen is present or absent, which provides variable control ofthe reverse bias voltage applied to the photo-detecting section 5.

This enables the necessary offset compensation to be made according tochanges in the environment through only a key operation on the inputsection 1.

As described above, with the present invention, it is possible to reducethe laborious task of shading light before measurement as found inconventional power meters, make the necessary offset compensation foroptical power level measurement automatically with a simpleconfiguration, and achieve highly-accurate measurements.

Additional embodiments of the present invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the present invention disclosed herein. It is intended thatthe specification and examples be considered as exemplary only, with thetrue scope of the present invention being indicated by the followingclaims.

We claim:
 1. A light measuring device comprising:a photo-detectingsection whose output current varies according to an applied reverse biasvoltage and which receives light from a measured object and outputs acurrent proportional to the intensity of the light; a bias section whichgenerates a first reverse voltage to prevent the output current of saidphoto-detecting section from flowing and a second reverse bias voltageto allow the output current of said photo-detecting section to flow andapplies the first reverse bias voltage and the second reverse biasvoltage to said photo-detecting section; a direct-current amplifyingsection which outputs a signal whose level is proportional to the outputcurrent from said photo-detecting section; and a processing sectionwhich determines the intensity of light from said measured object from afirst output from said direct-current amplifying section when said biassection is controlled so that said first reverse bias voltage may beapplied to said photo-detecting section and a second output from saiddirect-current amplifying section when said bias section is controlledso that said second reverse bias voltage may be applied to saidphoto-detecting section.
 2. A light measuring device according to claim1, wherein said bias section applies said first reverse bias voltage tosaid photo-detecting section to make the current multiplication factorof said photo-detecting section almost zero to prevent the outputcurrent of said photo-detecting section from flowing and also appliessaid second reverse bias voltage to said photo-detecting section to makethe current multiplication factor of said photo-detecting section one ormore to allow the output current of said photo-detecting section to flowfor measurement of the intensity of light from said measured object. 3.A light measuring device according to claim 1, wherein saidphoto-detecting section includes an InGaAs APD (Avalanche Photodiode).4. A light measuring device according to claim 3, wherein said firstreverse bias voltage said bias section applies to said photo-detectingsection is in the range from 0 V or higher to 13 V or lower and saidsecond reverse bias voltage is in the range from 16 V or higher to 100 Vor lower.
 5. A light measuring device according to claim 1, wherein saidprocessing section, when receiving an instruction to determine theintensity of light from said measured object, causes said bias sectionto apply said first reverse bias voltage to said photo-detecting sectionduring a first specific period for offset data acquisition, receivessaid first output from said direct-current amplifying section, andstores it as offset data,and after said specific period has elapsed,causes said bias section to apply said second reverse bias voltage tosaid photo-detecting section during a second specific period fordetermining the intensity of light from said measured object, receivessaid second output from said direct-current amplifying section, anddetermines the intensity of light from said measured object bysubtracting said offset data, more than once during said second specificperiod.